This invention relates to a process and apparatus for conserving water in a hot water supply system. More particularly, this invention relates to a process and apparatus for storing and subsequently recycling relatively cool water in the system which is located between a hot water heater and a point of use of hot water.
In typical hot water delivery systems having a water heater and a distal point of use (POU); intermediary piping between the heater and the POU most often contains relatively cool water which becomes cool due to heat energy exchange from the water to the atmosphere and structure surrounding the intermediary piping. As a result, it is common practice to open an outlet such as a faucet to move the cool water into an open sink until the hot water has reached the outlet. The amount of water wasted in this manner can be as high as three gallons or more per use of the hot water system. This problem is most pronounced in slab built homes, particularly in the South or Southwest United States that require long horizontal runs of piping between the water heater and the POU. The need for capturing the volume of cool water between the water heater and the POU has long been known. However, energy-efficient systems have not been available.
Systems that offer instant hot water at the POU are well known in larger buildings such as hotels and hospitals. Dedicated return lines allow the main hot water supply to be configured in continuous loops, requiring only short runs of piping between the heated trunk lines and the Points of Use. Similar systems are occasionally installed in domestic structures, but are expensive to build and operate.
There are a number of presently available systems that use existing, conventional plumbing to effect instant hot water (Imhoff, U.S. Pat. No. 5,009,572, Laing, U.S. Pat. No. 5,941,275). These operate by constantly pumping hot water to the point of use using the cold water supply as a return line. However, these systems require a number of compromises, including the following:
1. Because the hot water supply plumbing is kept hot at all times (as is, to a lesser degree, the cold water plumbing), a large amount of heat is lost from the pipes to the surrounding structure. Exacerbating this situation is that in warm weather locations, this waste heat must then be taken back out of the structure via air conditioning. A typical case study estimated that a constant re-circulation system saved $40 per year in water, but required an additional $200 of annual water heating energy and $300 of air conditioning.
2. Using the cold water supply as a return line results in an unstable supply temperature at the POU. Between uses, the hot water supply is rarely fully hot, and the cold water supply line will often be filled with lukewarm water. As a result, the mixing valve will require constant adjusting to maintain the desired outlet temperature until the cold water supply becomes completely cold and the hot water supply becomes completely hot.
3. Existing systems require AC electrical power to run pumps constantly. This requirement dramatically increases the cost of installation, and results in pumping costs that are comparable in cost to the expected water savings.
A number of systems have been disclosed that attempt to minimize the heat energy that is lost from hot supply plumbing. These systems transfer water from the cold supply to the hot supply before use, and transfer cold water back into the hot water supply after use. This forces the heated water back into the heater tank where it can be stored efficiently (Britt, U.S. Pat. No. 5,105,846, Lund, U.S. Pat. No. 5,277,219). However, such systems typically require large pumps to overcome dynamic head loss as water is moved through long runs of cold and hot water plumbing. Response time is typically slow, and installation and pumping costs can be expensive.
Holding tanks that are local to the POU and used to capture unwanted cooled water have also been proposed. A typical problem with POU holding tanks is that to get acceptable response time, the diverted water must be allowed to drop to near atmospheric pressure. Considerable pumping is then required to repressurize the water to the city supply pressure before it can be re-injected into the water heater inlet. Storch discloses a diverter valve and holding tank in U.S. Pat. No. 5,564,462. That system relies on conventional mechanical pumping to return the captured water to the water heater. In addition, it requires a dedicated return line to the water heater, rather than using the hot water supply line itself. In so doing, it loses the energy benefit of cooling the pipes after use.
In general, phase-change actuators are also well known. The earliest steam-powered machines utilized water as a working fluid in an open loop system. More recent designs such as the Solar Water Pump disclosed by O""Hare in U.S. Pat. No. 4,309,148 use a water vapor in a closed loop design with a diaphragm divider.
Actuators that utilize refrigerants and propellants are also well known. These fluids are often advantageous because they change phases at more convenient temperatures and pressures than does water. In U.S. Pat. No. 4,955,921 Basile discloses a toilet flushing mechanism that uses a propellant-filled bladder in a containment vessel for water. Rather than letting the tank water drop to atmospheric pressure upon refill, that system maintains the water at an elevated pressure using a pressurized bladder, thereby reducing the amount of water required per flush. Using a propellant in the bladder rather than air, the toilet tank volume is minimized. When water flows into the tank, the propellant in the bladder liquifies, allowing the bladder to shrink to a very small volume. During flushing, the fluid in the bladder expands back into its gaseous phase, maintaining the elevated pressure inside the reservoir and helping drive out the water for flushing. In this concept, no effort is made to vary bladder pressure by varying the temperature of the contained vapor.
In U.S. Pat. No. 4,070,859, Sobecks discloses a linear actuator that does vary the propellant temperature using a heating element. A rolling diaphragm is used to contain the propellant and transfer the resulting force to a spring-loaded shaft. Although such actuators can be inefficient and expensive, they allow accurate modulation of force, and were thus investigated by Chrysler for use in braking systems (Miesterfeld, U.S. Pat. No. 5,666,810).
It would be desirable to provide a process and apparatus for conserving cooled water in a hot water delivery system that includes a water heater and a control means, such as a faucet for delivering hot water to a point of use. In addition, it would be desirable to provide such a process and apparatus which minimizes heat loss from hot water supply plumbing. In addition, it would be desirable to provide such a process and apparatus which avoids the need for mechanical pumping means to pump water against a back pressure from a water source to the system, or overcome significant dynamic head loss due from moving water through long runs of piping. In addition, it would be desirable to provide such a process and apparatus which minimizes the time it takes hot water to be presented at the faucet.
The present invention provides a process and apparatus for conserving cool water located between a water heater and a point of use in a hot water delivery system. In addition, the system conserves heat energy by routing the unused hot water that is contained in the supply plumbing back to the water heater between uses. The system uses a holding tank at the Point of Use (POU) to capture and hold the otherwise-unused volume of cold water. Unique to this system is the dual-phase actuator that allows re-injection of the cooled water into the supply plumbing.
In accordance with this invention, the POU holding tank incorporates a flexible bladder or rolling diaphragm that separates the tank into a variable volume that can contain water from the hot water supply line, and another variable volume that contains a fixed amount of propellant or refrigerant in liquid and wet vapor phases.
At room temperature, the holding tank is at a low pressure relative to the supply plumbing, and provides a ready receptacle into which unwanted cooled water can be diverted by a control means. Said controller then utilizes a temperature sensor to determine when the hot water supply temperature has reached a predetermined setpoint, at which time the diverter valve is actuated to allow flow through to the POU. The process of introducing hot water to the POU also routes hot water through a heat-exchanging sump containing liquid propellant. Once the propellant is warmed by the adjacent hot water, the holding tank is repressurized to a pressure above that of the supply plumbing. At a predetermined later time, the diverter valve can be reopened, affecting a re-injection and xe2x80x9creplacementxe2x80x9d of the cool water slug back into the hot water supply line. The hot water is thus displaced back into the water heater, minimizing the heat loss from the hot water plumbing.
Note that most municipalities require backflow preventers (check valves) inline as the supply piping enters a structure. As a result, re-injecting a volume of water from a POU holding tank requires a buffer tank elsewhere in the system to accommodate the increase in total system volume. Such a tank can be easily added near the water heater inlet. The displaced hot water can be a) reinjected to the water heater outlet, forcing water out of the inlet and into the buffer tank, or b) reintroduced to the water heater inlet via the buffer tank. The insulated buffer tank would store the heated water efficiently, and at high pressure until the next requirement for hot water. This type of accumulator tank is well known in the hydraulic actuator design art.
The process and apparatus of this invention utilizes a phase change fluid. By xe2x80x9cphase change fluidxe2x80x9d as used herein is meant a composition that can be either in the liquid phase or in the gas phase at useful temperatures and pressures. In the present invention, it is preferred to utilize a fluid that is a liquid near room temperature and at atmospheric pressure. This minimizes the holding tank pressure during cold water capture, which minimizes tank-filling time, and improves system response time. Additionally, it is preferred that the vapor pressure of the fluid rise to 65 psig or more at 125xc2x0 F. or less. This is the pressure at which the cold water slug can be re-injected into the hot water supply line. Also, 125xc2x0 F. is a generally accepted safe temperature at which to maintain the hot water supply in households with children.
Compounds that exhibit these properties include Butane, as well as many safe propellants such as hexafluoropropane. These fluids are used for industrial and medical applications such as fire extinguishers and aerosol-delivered medications.
The process and apparatus of this invention will also result in a significant reduction in the time it takes hot water to be presented at the faucet. This is because the system can be located inline at a position before the primary restrictions in the supply line, which include the under-sink shutoff valve and the POU mixing valve. The system described here is retrofittable to all conventional plumbing configurations.